U.S. Pat. No. 3,344,923 describes a filter unit with two filter elements in the form of hollow cylinders. Both elements are of the same filtration grade. The filter elements are mounted in a common housing so that their longitudinal axes are in alignment. The two filter elements are separated from each other by a separating device extending transversely to the longitudinal axes. At the same time, the filter space (unfiltered fluid space) situated upstream from the filter materials in the direction of flow of the fluid to be cleaned is divided into two subspaces, between which a fluid-conducting connection may nevertheless be established by a valve installed in the separating device. This valve is a bypass valve and initially is closed. Opening of the valve occurs only when a certain pressure difference is present between the two subspaces as a result of increased fouling and accordingly decreasing permeability of the first filter element as viewed in the direction of fluid flow. The second filter element, which serves as reserve unit, can gradually assume the function of the first filter element to ensure uninterrupted trouble-free filtering operation. No provision is made for uninterrupted filtration operation with a primary flow and secondary flow filter of different filtration grades qualities.
GB 860 871 A discloses a generic filter device in which the fluid flow and accordingly the pressure relationships between the two subspaces associated with the first or second filter element are affected both in the unfiltered material space and on the filtrate space side. Here, the two filtrate partial flows emerging from the two filter elements are initially conducted separately by a tapering spiral spring whose narrower end is rigidly connected to the outlet opening for the partial flow of filtrate discharged from the finer, and thus, less permeable filter element. The wider free end of the spiral spring is loosely connected to the space surrounding it into which the partial filtrate flow of the coarser, and thus, more easily permeable filter element is discharged. The latter generates at the open end of the spiral spring vortices which exert a suction effect on the interior of the spiral spring. The influx of the filtrate partial flow from the finer filter element into the filtrate partial flow of the coarser filter element is consequently promoted. The suction effect increasing with increase in the fluid flow rate ultimately causes the turns of the spiral spring, previously closed in the direction of the longitudinal axis of this spring, to separate, so that the separation of the two filtrate partial flows along the spiral spring is eliminated and the pressure decrease accompanying the vortex formation is limited. When this conventional solution is applied, only the primary flow filter is in use when the fluid passage amounts are very low and very high. Only in the intermediate moderate fluid passage areas is filtration by the secondary filter mounted parallel to the primary filter initiated, something which is detrimental to the purity of the filtrate during filtration operation as a whole. No provision is made for use of the secondary filter during the filtration operation in question.